The present invention relates to injection molding machines and in particular to vacuum systems for such injection-molding machines.
Injection-molding injects heated thermoplastic material at high pressures from an injector into a mold. The thermoplastic material cools within the mold and the mold may be opened and the part ejected.
Gas trapped in the mold during the injection process may escape through the mold seams or through vents provided for that purpose. Alternatively, it is well known to apply a vacuum to the mold during the injection process to reduce gas entrapment. U.S. Pat. No. 4,573,900 entitled: Evacuation System for Injection-molding Machines, hereby incorporated by reference, describes a timer system controlling a pneumatic vacuum pump that may be used first, to apply a vacuum to the mold during certain periods of the injection cycle and second, to apply a positive pressure from the pneumatic line to the mold creating a “blow back” to assist in part ejection.
Standard machines normally employ proprietary control systems that do not provide control signals to activate the vacuum system at the proper time and duration. Accordingly, it is known in the art to attach mechanical limit switches to the mold and its associated clamping mechanism to create the necessary signals from mechanical movement of the mold elements.
This approach may not be suitable for creating more sophisticated vacuum control signals triggered by events or states not associated with a change in mold position, for example, as may occur in some two-shot molding applications. Modification of the standard injection-molding machine to attach the limit switches not envisioned by the manufacturer can be difficult and the limit switches, once in place, are subject to damage and contamination.